Humilisin E: Strategy for the Synthesis and Access to the Functionalized Bicyclic Core

Humilisin E is a diterpenoid possessing a rare epoxidized cyclononene trans-fused with a bicyclo[3.2.0]heptane core. We have identified the P atropisomer of the corresponding cyclononadiene as a potential biosynthetic/synthetic precursor to humilisin E and reported two different strategies for the stereocontrolled synthesis of the appropriately functionalized bicyclic cores of humilisin E. The first route involves a Stork epoxynitrile cyclization via a Mg alkoxide, and the second, more stereoselective approach utilizes the Wolff rearrangement as the key step.

H umilisins E and F [1a and 1b, respectively (Figure 1)] are novel terpenoids isolated from the South China Sea soft coral Sinularia humilis.They possess a highly substituted cyclobutane ring fused with cyclopentane and cyclononene ring systems. 1 Their structures also include an epoxide ring fused with the cyclononene ring (at C7−C8) and a hydroxy/ hydroperoxide group at C12.These functionalities may be introduced in late-stage oxidation processes from the terpene precursor during the biosynthesis of 1a and 1b.
From both synthetic and biosynthetic points of view, potential precursors might be the corresponding (3Z,7E)-dienes 2a and 2b (Figure 1).The P atropisomer of 2a possesses a configuration that leads to 1a via the epoxidation of the 7E alkenyl group.In (P)-2a, the si face of the (7E)-alkene is exposed, while the approach to the (3Z)-alkene is sterically hindered by the i-Pr group at C1 (Figure 1).This analysis suggests that late-stage regio-and stereoselective epoxidation of 2a or 2b to give humilisin E or F (1a or 1b), respectively, should be possible.Epoxidation of 2a and 2b represents a potential biosynthetic route to 1a and 1b, respectively.Preliminary density functional theory (DFT) calculations suggest that (P)-2a is thermodynamically more stable than (M)-2a by 14 kJ/mol (see the Supporting Information).
Synthetic access to (P)-2a requires the construction of a fused cyclononadiene.Synthesis of nine-membered rings is generally challenging, and in spite of their importance in natural products and medicinally important compounds, 2 their synthesis often requires special strategies such as conformational control to help the cyclization step. 3,4We propose that the conformational rigidity of the cyclobutane ring and the trans-disposed substituents at C2 and C10 of the cyclobutane could assist in the closure of the cyclononadiene by restricting the conformational freedom (Scheme 1).To test this hypothesis, we required access to the bicyclic cyclobutane−cyclopentane core of 1a and 2a [e.g., via 3 (Scheme 1)].Herein, we report the successful stereoselective synthesis of the functionalized bicyclo[3.2.0]heptane 5−7 subunit of humilisin E via two different approaches.

■ RESULTS AND DISCUSSION
Our first approach involved the epoxynitrile cyclization reported by Stork, which provides a unique way to synthesize functionally substituted cyclobutane rings in a sterically challenging environment. 8The plan toward (±)-4a involved the construction of a heavily functionalized cyclopentane via sequential conjugate addition and alkylation sequence.The route commenced with β-substituted enone ( 6), available from 1,3diketone (5) via enol ether formation followed by Grignard addition. 4,9A Cu-catalyzed conjugate addition of vinyl magnesium bromide to enone 6 in the presence of TMSCl led to the isolation of silyl enol ether (±)-7 in quantitative yield.The introduction of the nitrile functionality via alkylation of (±)-7 with bromoacetonitrile 11−13 via in situ generation of the enolate with KOtBu or CsF initially failed, resulting in either recovery of (±)-7 (CsF) or the corresponding ketone (KOtBu).Lewis acids [Yb(OTf) 3 , InCl 3 , and BF 3 ] also failed to promote the alkylation.However, the lithium enolate was readily generated by MeLi at 0 °C, and by subsequent alkylation with bromoacetonitrile at −50 to −30 °C, the desired alkylation product (±)-8 was obtained in 30% yield and 9:1 dr.Unfortunately, even after extensive changes in the variables, (temperatures, solvents, reaction times, and concentrations), the yield of the alkylation reaction could not be improved, Nevertheless, the addition of MeMgBr to ketone (±)-8 furnished tertiary alcohol (±)-9 with excellent diastereoselectivity (>20:1 dr).
Computational analysis of the desired isomer, (±)-11, revealed a potential risk in the synthesis plan, which was not anticipated earlier.The epoxy nitrile cyclization with a base might also rapidly deprotonate the tertiary alcohol, resulting in the undesired 6-endo (or 5-exo) cyclization with the epoxide forming bicyclic ether, as the C12 OH and the epoxide were both pseudoaxially disposed in (±)-10.To guard against this liability, attempts were made to protect the tertiary alcohol with a TMS group, but without success.Therefore, to avoid the unwanted cyclization or alkylation of the C12 tertiary alcohol, we followed a literature precedent by Fleming and co-workers, who had used Grignard reagents to metalate nitriles and simultaneously form Mg alkoxides from tertiary alcohols. 14,15e hypothesized that the formation of Mg alkoxide would deactivate the tertiary alcohol.Indeed, epoxynitrile (±)-11 was successfully cyclized to afford cyclobutane (±)-4a upon treatment with i-PrMgCl in 30% yield.The relative stereochemistry of (±)-4a was confirmed by nuclear Overhauser effect (NOE) experiments (Scheme 2) as well as by comparison of the 1 H NMR coupling constants (see Figure 2).Again, attempts to improve the yield of this step were unsuccessful; with a larger excess of i-PrMgCl (6 equiv), addition to the nitrile was also observed, resulting in the formation of the corresponding isopropyl ketone in <25% yield.Decreasing the temperature did not improve the chemoselectivity; no reaction was observed at 0 °C.Prolonged reaction times resulted in decomposition.

The Journal of Organic Chemistry
Although the first-generation synthesis of the bicyclic core delivered products with the desired relative stereochemistry, the route suffered from a number of low-yielding steps and poor stereocontrol.We therefore started over and outlined a second strategy involving Wolff ring contraction to construct an appropriately functionalized cyclobutane ring (Scheme 3).
The second strategy was designed to avoid stereocontrol issues by constructing the bicyclic system earlier in the route, followed by ring contraction of the bicyclo[3.3.0]octanering system to the desired bicyclo[3.2.0]heptane.

■ CONCLUSION
In conclusion, we have developed two alternative routes to the functionalized bicyclo[3.2.0]heptane core of humilisin E via either the Stork nitrile epoxide method or Wolff rearrangement. 31The asymmetric version of the second route and progress in the total synthesis of 1a will be reported in due course.

Scheme 1 .
Scheme 1. Retrosynthetic Analysis of 2 via Conformationally Restricted Intermediate 3 a and Key Core Fragments 4a and 4b